By virtue of its high level of crystallinity, trans-1,4-polybutadiene (TPBD) is typically a thermoplastic resin. Because it contains many double bonds in its polymeric backbone, TPBD can be blended and cocured with rubber. TPBD is similar to syndiotactic-1,2-polybutadiene in this respect. Even though trans-1,4-polybutadiene having a high melting point is a thermoplastic resin, it becomes elastomeric when cured alone or when cocured with one or more rubbers.
Good molecular weight control can normally be achieved by utilizing an anionic polymerization system to produce TPBD. There is typically an inverse relationship between the catalyst level utilized and the molecular weight attained when anionic polymerization systems are used. Such an anionic polymerization system is disclosed in U.S. Pat. No. 4,225,690. The catalyst system disclosed therein is based on a dialkylmagnesium compound which is activated with a potassium alkoxide. However, such catalyst systems have not proven to be commercially successful.
TPBD is normally prepared utilizing transition metal catalysts or rare earth catalysts. The synthesis of TPBD with transition metal catalysts is described by J. Boor Jr., xe2x80x9cZiegler-Natta Catalysts and Polymerizations,xe2x80x9d Academic Press, New York, 1979, Chapters 5-6. The synthesis of TPBD with rare earth catalysts is described by D. K. Jenkins, Polymer, 26, 147 (1985). However, molecular weight control is difficult to achieve with such transition metal or rare earth catalysts and monomer conversions are often very modest.
Japanese Patent Application No. 67187-1967 discloses a catalyst system and technique for synthesizing TPBD consisting of 75 to 80 percent trans-1,4-structure and 20 to 25 percent 1,2-structure. The catalyst system described by this reference consists of a cobalt compound having a cobalt organic acid salt or organic ligand, an organoaluminum compound and phenol or naphthol. Gel formation is a serious problem that is frequently encountered when this three-component catalyst system is utilized in the synthesis of TPBD. Gelation is a particularly serious problem in continuous polymerizations. By utilizing this catalyst system and technique, TPBD can be synthesized in a continuous process with only minimal amounts of gel formation.
U.S. Pat No. 5,089,574 is based upon the finding that carbon disulfide will act as a gel inhibitor in conjunction with three component catalyst systems which contain an organocobalt compound, an organoaluminum compound and a para-alkyl substituted phenol. U.S. Pat. No. 5,089,574 also indicates that conversions can be substantially improved by utilizing para-alkyl substituted phenols which contain from about 12 to about 26 carbon atoms and which preferably contain from about 6 to about 20 carbon atoms.
U.S. Pat. No. 5,089,574 more specifically reveals a process for synthesizing trans-1,4-polybutadiene in a continuous process which comprises continuously charging 1,3-butadiene monomer, an organocobalt compound, an organoaluminum compound, a para-substituted phenol, carbon disulfide and an organic solvent into a reaction zone; allowing the 1,3-butadiene monomer to polymerize in said reaction zone to form the trans-1,4-polybutadiene; and continuously withdrawing the trans-1,4-polybutadiene from said reaction zone.
U.S. Pat. No. 5,448,002 discloses that dialkyl sulfoxides, diaryl sulfoxides and dialkaryl sulfoxides act as molecular weight regulators when utilized in conjunction with cobalt-based catalyst systems in the polymerization of 1,3-butadiene monomer into TPBD. U.S. Pat. No. 5,448,002 reports that the molecular weight of the TPBD produced decreases with increasing levels of the dialkyl sulfoxide, diaryl sulfoxide or dialkaryl sulfoxide present as a molecular weight regulator.
U.S. Pat. No. 5,448,002 specifically discloses a process for the synthesis of trans-1,4- polybutadiene which comprises polymerizing 1,3- butadiene monomer under solution polymerization conditions in the presence of at least one sulfoxide compound selected from the group consisting of dialkyl sulfoxides, diaryl sulfoxides and dialkaryl sulfoxides as a molecular weight regulator and in the presence of a catalyst system which includes an organocobalt compound, an organoaluminum compound and a para-alkyl substituted phenol.
The presence of residual cobalt in TPBD made with cobalt-based catalyst systems is not desirable. This is because the residual cobalt acts as a prooxidant leading to polymer instability during storage. This is a particular problem in cases where the TPBD is stored in a xe2x80x9chothousexe2x80x9d prior to usage, which is a standard procedure in many industries, such as the tire industry. In any case, high levels of residual cobalt in the TPBD lead to problems with polymer stability.
Unfortunately, carbon disulfide is typically required as a gel-reducing agent in the synthesis of TPBD with cobalt-based catalyst systems. This is particularly true in the case of continuous polymerization systems. However, the presence of carbon disulfide in such systems reduces the level of catalyst activity and generally makes it necessary to increase the level of cobalt in the catalyst system. Thus, in cases where carbon disulfide is required for gel control, the level of cobalt needed is further increased. This accordingly leads to greater polymer instability.
Due to its high melting point, it is normally necessary to heat TPBD in order for it to be processed using conventional mixing equipment, such as a Banbury mixer or a mill mixer. This heating step is typically carried out by storing the trans-1,4-polybutadiene in a xe2x80x9chot-housexe2x80x9d for a few days prior to its usage. During this storage period, the bails of the polymer are slowly heated to a temperature above about 104xc2x0 F. (40xc2x0 C.). At such temperatures, the polymer can be readily processed in standard mixing equipment. However, the TPBD typically undergoes undesirable oxidative crosslinking which leads to gelation during this long heating period. This oxidation can crosslink the TPBD to such a high degree that it cannot be processed utilizing standard mixing techniques. In other words, the oxidative gelation that occurs can destroy the polymer.
U.S. Pat. No. 5,854,351 discloses that TPBD which contains a processing oil can be rapidly heated by radio frequency electromagnetic radiation. The radio frequency waves used in such a heating process typically have a frequency that is within the range of about 2 to 80 MHz (megahertz). By utilizing such a technique, an 80-pound (30 kg) bail of TPBD can be rapidly heated to a temperature above 104xc2x0 F. (40xc2x0 C.) in a matter of minutes. During this rapid heating process, oxidative gelation does not occur to a significant degree. This is, of course, in contrast to conventional heating techniques where bails of TPBD are slowly warmed by convection heating to the required temperature over a period of days. During this long heating period, the TPBD undergoes highly undesirable oxidative crosslinking.
U.S. Pat. No. 5,854,351 more specifically discloses a technique for mixing trans-1,4- polybutadiene with at least one rubbery polymer which comprises: (1) heating the trans-1,4-polybutadiene to a temperature which is within the range of 105xc2x0 F. (41xc2x0 C.) to 200xc2x0 F.(93xc2x0 C.) by exposing it to electromagnetic radiation having a frequency in the range of about 2 MHz to about 80 MHz, wherein the trans-1,4-polybutadiene is oil-extended with at least 10 phr of a processing oil; and (2) mixing the trans-1,4-polybutadiene with said rubbery polymer before any portion of the trans-1,4-polybutadiene cools to a temperature below 104 F.xc2x0 (41xc2x0 C.).
U.S. Pat. No. 5,100,965 discloses a process for synthesizing a high trans polymer which comprises adding (a) at least one organolithium initiator, (b) an organoaluminum compound, (c) a barium alkoxide and (d) a lithium alkoxide to a polymerization medium which is comprised of an organic solvent and at least one conjugated diene monomer.
U.S. Pat. No. 5,100,965 further discloses that high trans polymers can be utilized to improve the characteristics of tire tread rubber compounds. By utilizing high trans polymers in tire tread rubber compounds, tires having improved wear characteristics, tear resistance and low temperature performance can be made.
In commercial applications where recycle is required, the use of barium alkoxides can lead to certain problems. For instance, barium t-amylate can react with water to form t-amyl alcohol during steam-stripping in the polymer finishing step. Since t-amyl alcohol forms an azeotrope with hexane, it co-distills with hexane and thus contaminates the feed stream.
This invention is based upon the discovery that the problem of recycle stream contamination can be solved by synthesizing trans-1,4-polybutadiene utilizing a catalyst system which is comprised of (a) an organolithium compound, (b) a barium compound selected from the group consisting of (i) barium salts of cyclic alcohols, such as barium mentholate, and (ii) barium thymolate, and (c) an organoaluminum compound. The problem of recycle stream contamination is solved by utilizing a barium salt of a cyclic alcohol as the barium compound in the catalyst system. Barium mentholate is highly preferred because it does not co-distill with hexane or form compounds during steam-stripping which co-distill with hexane. Since the boiling points of the cyclic alcohols generated upon the hydrolysis of their metal salts are very high, they do not co-distill with hexane and contaminate recycle streams. Additionally, such cyclic alcohols are considered to be environmentally safe. In fact, menthol (the hydrolyzed product of barium mentholate) is commonly used as a food additive.
The trans-1,4-polybutadiene made with such barium containing catalyst systems has a melting point that is within the range of about -30xc2x0 C. to +30xc2x0 C. Because the trans-1,4-polybutadiene synthesized with the catalyst system of this invention has a high melting point it does not need to be heated in a xe2x80x9chot-housexe2x80x9d before it is blended with other rubbery polymers or utilized in making rubber products, such as tires. Additionally, the trans-1,4-polybutadiene is strain crystallizable and can be employed in manufacturing tire tread compounds that exhibit wear characteristics. The trans-1,4-polybutadiene also typically has a glass transition temperature which is within the range of about -97xc2x0 C. to about xe2x88x9290xc2x0 C., a number average molecular weight which is within the range of about 50,000 to about 400,000, and a Mooney ML 1+4 viscosity which is within the range of about 5 to about 110.
The present invention more specifically discloses a process for synthesizing trans-1,4-polybutadiene which comprises polymerizing 1,3-butadiene monomer in the presence of a catalyst system which is comprised of (a) an organolithium compound, (b) a barium compound selected from the group consisting of (i) a barium salt of a cyclic alcohol, and (ii) barium thymolate, and (c) an organoaluminum compound.
The present invention further discloses a process for synthesizing trans-1,4-polybutadiene which comprises polymerizing 1,3-butadiene monomer in the presence of a catalyst system which is comprised of (a) an organolithium compound, (b) a barium compound selected from the group consisting of (i) a barium salt of a cyclic alcohol, and (ii) barium thymolate, (c) an organoaluminum compound, and (d) a lithium salt of a cyclic alcohol.